This invention relates generally to overcoatings for ionographic or electrophotographic imaging and printing apparatuses and machines, and more particularly is directed to an effective overcoating for a donor means like a roll, preferably with electrodes closely spaced therein to form a toner cloud in the development zone to develop a latent image. The present invention is directed, in embodiments, to suitable conductive and semiconductive overcoatings especially for the donor roll or transport means in systems like scavengeless or hybrid scavengeless development systems, reference for example U.S. Pat. No. 4,868,600, U.S. Pat. No. 5,172, 170, and copending patent applications U.S. Ser. No. 396,153 (now abandoned) and U.S. Ser. No. 724,242 now abandoned, the disclosures of which are totally incorporated herein by reference.
Overcoatings for donor rolls are known and can contain a dispersion of conductive particles, like carbon black, or graphite in a dielectric binder, such as a phenolic resin or fluoropolymer, as disclosed in U.S. Pat. No. 4,505,573. The dielectric constant of the overcoatings ranges from about 3 to about 5, and preferably is about 3, and the desired resistivity is achieved by controlling the loading of the conductive material. However, very small changes in the loading of conducive materials near the percolation threshold can cause dramatic changes in resistivity. Furthermore, changes in the particle size and shape of such materials can cause wide variations in the resistivity at constant weight loading. A desired volume electrical resistivity of the overcoating layer is in the range of from about 10.sup.7 ohm-cm to about 10.sup.13 ohm-cm, and preferably, the electrical resistivity is in the range of 10.sup.8 ohm-cm to about 10.sup.11 ohm-cm. If the resistivity is too low, electrical breakdown of the coating can occur when a voltage is applied to an electrode or material in contact with the overcoating. Also, resistive heating can cause the formation of holes in the coating. When the resistivity is too high, for example about 10.sup.13 ohm-cm, charge accumulation on the surface of the overcoating creates a voltage which changes the electrostatic forces acting on the toner. The problem of the sensitivity of the resistivity to the loading of conductive materials in an insulative dielectric binder is avoided, or minimized with the coatings of the present invention.
Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image of an original document being reproduced. This records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two component and single component developer materials are commonly used for development. A typical two component developer comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single component developer material typically comprises toner particles. Toner particles are attracted to the latent image forming a toner powder image on the photoconductive surface, the toner powder image is subsequently transferred to a copy sheet, and finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.
Trilevel, highlight color xerography is described, for example, in U.S. Pat. No. 4,078,929 (Gundlach). This patent discloses trilevel xerography as a means to achieve single-pass highlight color imaging wherein a charge pattern is developed with toner particles of a first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the second color are negatively charged. In one embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brush development systems wherein each system supplies a toner of one color and one charge.
In highlight color xerography (Gundlach), the xerographic contrast on the charge retentive surface or photoreceptor is divided into three levels, rather than two levels as is the situation for conventional xerography. The photoreceptor is charged, typically to -900 volts, and is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, CAD) remains at the full photoreceptor potential (V.sub.cad or V.sub.ddp). The other image is exposed to discharge the photoreceptor to its residual potential, for example V.sub.dad or V.sub.c (typically -100 volts), which corresponds to discharged area images that are subsequently developed by discharged area development (DAD) and the background areas exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically -500 volts) and is referred to as V.sub.white or V.sub.w. The CAD developer is typically biased about 100 volts closer to V.sub.cad than V.sub.white (about -600 volts), and the DAD developer system is biased about 100 watts closer to V.sub.dad than V.sub.white (about -400 volts).
The viability of printing system concepts such as trilevel and highlight color xerography usually requires development systems that do not scavenge or interact with a previously toned image. Since several known development systems, such as conventional magnetic brush development and jumping single component development, interact with the image receiver, a previously toned image will be scavenged by subsequent development, and as these development systems are highly interactive with the image bearing member, there is a need for scavengeless or non interactive development systems.
Single component development systems use a donor roll for transporting charged toner to the development nip defined by the donor roll and photoconductive member. The toner is developed on the latent image recorded on the photoconductive member by a combination of mechanical and/or electrical forces. Scavengeless development and jumping development are two types of single component development systems that can be selected. In one version of a scavengeless development system, a plurality of electrode wires are closely spaced from the toned donor roll in the development zone. An AC voltage is applied to the wires to generate a toner cloud in the development zone. The electrostatic fields associated with the latent image attract toner from the toner cloud to develop the latent image. In another version of scavengeless development, isolated electrodes are provided within the surface of a donor roll. The application of an AC bias to the electrodes in the development zone causes the generation of a toner cloud. In jumping development, an AC voltage is applied to the donor roll for detaching toner from the donor roll and projecting the toner toward the photoconductive member so that the electrostatic fields associated with the latent image attract the toner to develop the latent image. Single component development systems appear to offer advantages in low cost and design simplicity. However, the achievement of high reliability and simple, economic manufacturability of the system continue to present problems. Two component development systems have been used extensively in many different types of printing machines.
A two component development system usually employs a magnetic brush developer roller for transporting carrier having toner adhering triboelectrically thereto. The electrostatic fields associated with the latent image attract the toner from the carrier so as to develop the latent image. In high speed commercial printing machines, a two component development system may have lower operating costs than a single component development system. Clearly, two component development systems and single component development systems each have their own advantages. Accordingly, it is considered desirable to combine these systems to form a hybrid development system having the desirable features of each system. For example, at the 2nd International Congress on Advances in Non-Impact Printing held in Washington, D.C. on Nov. 4 to 8, 1984, sponsored by the Society for Photographic Scientists and Engineers, there was described a development system using a donor roll and a magnetic roller. The donor roll and magnetic roller were electrically biased. The magnetic roller transported a two component developer material to the nip defined by the donor roll and magnetic roller, and toner is attracted to the donor roll from the magnetic roll. The donor roll is rotated synchronously with the photoconductive drum with the gap therebetween being about 0.20 millimeter. The large difference in potential between the donor roll and latent image recorded on the photoconductive drum causes the toner to jump across the gap from the donor roll to the latent image and thereby develop the latent image.